Nuclear magnetic resonance (NMR), or magnetic resonance spectroscopy, is a powerful, well-established tool for studying chemical samples and sample interactions. In NMR, the spin and magnetism of atomic nuclei are exploited to provide information about the chemical composition, spatial distribution, or molecular motion of molecules or atoms. The imaging analog of NMR, magnetic resonance imaging (MRI), is a powerful technique in biomedical sample imaging.
One of the limitations of NMR and MRI is low intrinsic signal strength. Some attempts to overcome this limitation have involved the use of hyperpolarized contrast agents, which have very large nuclear polarizations and, therefore, sensitivities that are orders of magnitude higher than ordinary molecules. For a few molecules, polarization can persist for 100 or more seconds before the polarized nuclei return to thermal equilibrium. However, for the majority of molecules, including those that could be used as in vivo contrast agents for MRI, polarization lasts more in the range of seconds or tens of seconds.
While such lifetimes can be sufficient for some imaging and/or spectroscopy studies, contrast agents with longer lifetimes are highly desirable to study additional processes of interest, for example processes related to diffusion, flow, slow molecular motion, chemical reactions, metabolism, and drug targeting and distribution, among others. The relaxation of nuclear spins back to thermal equilibrium is characterized by a time constant, T1, known as the longitudinal relaxation time constant or as the spin lattice relaxation time constant. The development of contrast agents having polarization that persists for times longer than T1 would be beneficial for both NMR and MRI.